The present invention relates to a new process for producing aliphatic oligocarbonate diols by the transesterification of aliphatic diols with dimethyl carbonate (DMC) under elevated pressure. The process according to the invention also makes it possible to produce aliphatic oligocarbonate diols on a large industrial scale and with a high space-time yield (STY) from readily available DMC.
Aliphatic oligocarbonate diols are important precursors for the production of plastics, lacquers and adhesives, for example. They are reacted with isocyanates, epoxides, (cyclic) esters, acids or acid anhydrides, for example. They can be produced from aliphatic diols by the reaction thereof with phosgene (e.g. DE-A 1 595 446), esters of bis-chlorocarbonic acid (e.g. DE-A 0 857 948), diaryl carbonates (e.g. DE-A 1 915 908), cyclic carbonates (e.g. DE-A 2 523 352: ethylene carbonate) or dialkyl carbonates (e.g. DE-A 2 555 805).
Of the carbonate sources, diphenyl carbonate (DPC), which is a diaryl carbonate, is particularly important, since aliphatic oligocarbonate diols of particularly high quality can be produced from DPC (e.g. U.S. Pat. No. 3,544,524, EP-A 0 292 772). In contrast to all other carbonate sources, DPC reacts quantitatively with aliphatic OH functions, so that after removing the phenol which is formed, all the terminal OH groups of the oligocarbonate diol are available for reaction, e.g. with isocyanate groups. Moreover, only very low concentrations of a soluble catalyst are required, so that the latter can remain in the product.
Processes based on DPC have the following disadvantages, however:
Only about 13% by weight of the DPC remains as CO groups in the oligocarbonate; the remainder is distilled off as phenol. A significantly higher proportion of dialkyl carbonates remains in the oligocarbonate, depending on the alkyl radical concerned. Thus about 31% by weight of dimethyl carbonate (DMC) is available as CO for the oligocarbonate, since the methanol which is distilled off has a molecular weight which is considerably lower than that of phenol.
Because high-boiling phenol (normal boiling point: 182xc2x0 C.) has to be separated from the reaction mixture, it is only diols with a boiling point considerably higher than 182xc2x0 C. which can be used in the reaction, in order to prevent unwanted removal of the diol by distillation.
Due to their ease of production, dialkyl carbonates, particularly dimethyl carbonate (DMC), are distinguished as starting materials by being more readily available. For example, DMC can be obtained by direct synthesis from MeOH and CO (e.g. EP-A 0 534 454, DE-A 19 510 909).
Numerous publications (e.g. U.S. Pat. Nos. 2,210,817, 2,787,632, EP-A 364 052) relate to the reaction of dialkyl carbonates with aliphatic diols:
In the prior art, aliphatic diols are placed in a vessel together with a catalyst and the dialkyl carbonate (e.g. diethyl carbonate, dibutyl carbonate, diallyl carbonate), and the resulting alcohol (e.g. ethanol, butanol, allyl alcohol) is distilled off from the reaction vessel via a column. In the column, the higher boiling, dialkyl carbonate is separated from the lower boiling alcohol and is recycled to the reaction mixture.
Despite its ready availability, the use of dimethyl carbonate (DMC) for the production of aliphatic oligocarbonate diols has only recently become known (e.g. U.S. Pat. No. 5,171,830, EP-A 798 327, EP-A 0 798 328, DE-A 198 29 593).
EP-A 0 798 328 describes the reaction of the corresponding diol component with DMC with distillation of the azeotrope under normal pressure. Uncapping is subsequently effected by vacuum distillation, wherein degrees of utilization of the terminal OH groups of about 98% are achieved under very drastic vacuum conditions (1 torr, about 1.3 mbar) (EP-A 0 798 328: Table 1).
EP-A 0 798 327 describes a corresponding two-step process in which a diol is first reacted with an excess of DMC, with distillation of the azeotrope under normal pressure, to form an oligocarbonate, the terminal OH groups of which are present as methoxycarbonyl terminal groups and are completely inaccessible. After removing the catalyst and distillation of the excess DMC under vacuum (65 torr, 86 mbar) the oligocarbonate diol is obtained in a second step by the addition of further amounts of the diol and of a solvent (e.g. toluene) as an entraining agent for the methanol formed. The remainder of the solvent then has to be distilled off under vacuum (50 torr, 67 mbar). The disadvantages of this process are the cost of conducting it by the use of a solvent, and the repeated distillation which is required, as well as the very high consumption of DMC.
DE-A 198 29 593 teaches the reaction of a diol with DMC, with the methanol formed being distilled off under normal pressure. Apart from a single mention of the word xe2x80x9cazeotropexe2x80x9d in the table headed xe2x80x9cProcess diagram of the process according to the inventionxe2x80x9d, no consideration is given there to the overall problem of the azeotrope. It can be calculated from the examples that DMC is used in excess and is azeotropically distilled off. About 27.8% by weight of the DMC used is lost.
According to U.S. Pat. No. 5,171,830, a diol is first heated with DMC and volatile constituents are then (azeotropically) distilled off. After vacuum distillation under drastic conditions (1 torr, 1.3 mbar), take-up of the product in chloroform, precipitation of the product with methanol and drying the product, an oligocarbonate diol is obtained in a yield of 55% by weight theoretical (loc. cit., Example 6). The degree of utilization of the terminal OH groups and the azeotrope problems are not considered in detail. Although U.S. Pat. No. 5,171,830 mentions, in column 5, lines 24 to 26, that the process can be conducted under vacuum, at normal pressure and at elevated pressures, and therefore can be conducted under all pressures, the particular preferences regarding the conditions of pressure employed cannot be identified. It is only a procedure which employs reduced pressure for the removal of volatile constituents which is mentioned.
Therefore, in the above publications, which were known hitherto, there is no description of a process, which is simple to carry out industrially, for the reaction of DMC with aliphatic diols to form oligocarbonate diols with high space-time yields and with high degrees of utilization of the terminal OH groups.
It is an object of the present invention to provide a simple, productive process, which can also be carried out on a large industrial scale, and which enables oligocarbonate diols to be produced by the transesterification of aliphatic diols with dimethyl carbonate, optionally with the use of an amount of catalyst which is low enough for the latter to remain in the product after completion of the reaction, with good space-time yields and with a high degree of uncapping of the terminal OH groups, in simple apparatuses.
It has now been found that the production of aliphatic oligocarbonate diols by the reaction of aliphatic diols with dimethyl carbonate, with the reaction optionally being accelerated by catalysts, at elevated pressure, results in a high space-time yield. In order to complete the reaction and in order to uncap the terminal OH groups (render the latter utilizable), the final residues of methanol and traces of dimethyl carbonate are removed from the product under reduced pressure, optionally with the introduction of inert gas.
The invention relates to a process for producing an aliphatic oligocarbonate diol comprising a) reacting an aliphatic diol with dimethyl carbonate at an elevated pressure, and b) subsequently removing unreacted methanol and dimethyl carbonate under a reduced pressure to uncap the terminal OH groups.
The present invention also relates to a process of making a polymeric material comprising reacting the oligocarbonate diol.
The process according to the invention is conducted under elevated pressure, preferably under a pressure of 1.5 to 100 bar and more preferably under a pressure of 3 to 16 bar andxe2x80x94depending on the pressure employedxe2x80x94at temperatures from 100 to 300xc2x0 C., preferably at temperatures from 160 to 240xc2x0 C.
At a constant catalyst concentration, an elevated pressure results in a better conversion of DMC and in a shortening of the reaction times, which has a positive effect on the space-time yield.
Completion of the reaction and uncapping of the terminal OH groups (rendering the latter utilizable) are achieved by removing the final residues of methanol and traces of dimethyl carbonate under reduced pressure. In one preferred embodiment, completion the reaction and uncapping of the terminal OH groups (rendering the latter utilizable) are effected by introducing an inert gas (e.g. N2) into the oligocarbonate diol under what is only a slight vacuum of about 150 mbar. The gas bubbles are saturated with methanol or DMC and the methanol is thus almost completely expelled from the reaction batch. By stripping with an inert gas to remove methanol, the equilibrium can be further displaced in favour of the product, the transesterification is completed and the terminal groups are thus rendered utilizable. The quality of the resulting oligocarbonate diol can be raised to the level of f DPC-based oligocarbonate diols, and the degree of uncapping of the terminal OH groups increases to more than 98%, preferably to 99.0 to 99.95%, most preferably to 99.5 to 99.9%.
Gas bubbles can be produced by introducing inert gases such as nitrogen, noble gases such as argon, or methane, ethane, propane, butane, dimethyl ether, dry natural gas or dry hydrogen into the reactor, wherein part of the gas stream which leaves the oligocarbonate and which contains methanol and dimethyl carbonate can be rerouted to the reaction of the oligocarbonate for completion of the reaction. Nitrogen is preferably used. Air can be used if products of low standard with respect to color are to be made.
Gas bubbles can also be produced by introducing inert, low boiling liquids such as pentane, cyclopentane, hexane, cyclohexane, petroleum ether, diethyl ether or methyl tert-butyl ether, etc., wherein these substances can be introduced in liquid or gaseous form, and part of the gas stream which leaves the oligocarbonate and which contains methanol and dimethyl carbonate can be recycled to the oligocarbonate for saturation.
The substances for producing gas bubbles can be introduced into the oligocarbonate via simple immersion tubes, preferably by means of annular nozzles or gasification agitators. The degree of utilization of the terminal OH groups which is achieved depends on the duration of uncapping, and on the amount, size and distribution of the gas bubbles: with increasing duration of uncapping and better distribution (e.g. better distribution and a larger phase boundary, due to a larger number of smaller gas bubbles when the latter are introduced via a gasification agitator) the degree of utilization is better. When introducing nitrogen, for example (e.g. at 150 mbar, 8 kettle volumina/hour), using a gasification agitator, a degree of utilization of about 99% is achieved after one hour, and a degree of utilization of about 99.8% is achieved after about 5 to 10 hours.
Uncapping, optionally assisted by the introduction of inert gas bubbles into the reaction mixture, is conducted at temperatures from 160xc2x0 C. to 250xc2x0 C., preferably at temperatures from 200xc2x0 C. to 240xc2x0 C., and under pressures from 1 to 1000 mbar, preferably under pressures from 30 to 400 mbar, most preferably under pressures from 70 to 200 mbar.
During the production of oligocarbonate diols, DMC is distilled off during the production process. The amount of DMC which has been removed by distillation from the reaction batch is determined by determining the DMC content of the distillate. This missing amount has to be made up before stripping off the methanol with inert gases under vacuum to make the terminal groups utilizable. A mixture of DMC and methanol forms again. The DMC which is lost by the stripping can be added again, and another part is distilled off again. With each addition the amount of DMC which is distilled off becomes less, and the desired stoichiometry is thereby approached.
This costly procedure can be simplified by combining the individual addition steps. The amounts of DMC which were distilled off can be measured in previous batches made with individual addition steps. It is therefore possible subsequently to add the complete amount of DMC together in a single step.
Thus the total amount of DMC required, namely the sum of the amount which is predetermined by the stoichiometry of the desired product together with the amount of DMC which is distilled off whilst the reaction is conducted, is added directly in the first step.
During the distillation of the methanol and the uncapping of the OH terminal groups at the end of the reaction when inert gas bubbles are introduced, a small amount of DMC is lost. This amount has to be taken into account beforehand by the addition of DMC. The requisite amount can be determined from previous batches, based on experience.
In one preferred process variant, an excess of DMC is added at the start of the reaction which is calculated so that after distilling off the azeotrope and after uncapping, a product is formed which comprises the complete functionality of the terminal OH groups, but which has a degree of polymerisation which is too high. A correction is then made by adding a further amount of the diol component and by conducting a brief transesterification step again. The correction amount can firstly be determined via the mass balancexe2x80x94by determining the amount of DMC in all the distillates and making a comparison with the total amount addedxe2x80x94or from a measurable property (e.g. OH number, viscosity, average molecular weight, etc.) of the product, the degree of polymerisation of which is too high. Renewed uncapping is not necessary after this correction, since all the terminal OH groups are already freely available before the correction, and the addition of the diol components does not result in renewed capping.
Correction by the addition of DMC, after uncapping by gasification with an inert gas for a product which contains too little DMC, results in renewed capping.
According to the invention, the diols and optionally the catalysts which are present, are placed in a reaction vessel, the reactor is heated, the pressure is applied and DMC is subsequently metered in.
In one embodiment of the invention the process according to the invention therefore comprises the following steps:
Placing the diol components and optionally the catalyst in a vessel.
Heating and application of pressure.
Introduction and reaction of the DMC. The amount of DMC is calculated so that after removal by distillation in all steps (addition of DMC and uncapping) it is just the requisite amount of DMC or alternatively a slight excess thereof which remains in the reaction solution. Metered addition can be conducted according to two different strategies:
a) The complete amount of DMC is metered in rapidly in one step. As a consequence, the STY is optimised. A DMC-methanol mixture is distilled off which has a relatively high DMC content (e.g. the azeotrope), which is considerably less than that obtained in a pressureless procedure.
b) The DMC is metered in in two partial steps. The DMC is first metered in slowly, so that DMC-methanol mixtures with low DMC contents are distilled off. Not until a later point in time, when the DMC content in the distillate significantly increases, even at the same slow rate of addition, is the DMC rapidly metered in, so that a distillate with a high DMC content (e.g. a DMC-methanol azeotrope) is formed.
Procedure b) results in better utilization of the DMC and in an inferior STY.
Uncapping: rendering the terminal OH groups utilizable by extracting the final residues of methanol and DMC under reduced pressure, optionally by the production of gas bubbles (e.g. by the introduction of inert gases such as N2).
Correction: correction of the stoichiometry, if necessary, by adding further amounts of the diol components and another brief transesterification.
It is also possible, of course, for the process according to the invention to be conducted with an excess of diol. In a procedure of this type, a correction subsequently has to be made wit DMC. This then results in a repeated uncapping step.
In a further embodiment of the invention up to 100%, preferably up to 70%, more preferably up to 50% and most preferably up to 30% of the DMC is placed in the reaction vessel at the start, together with the diols and the catalyst which is optionally present. The reactor is subsequently closed, heated and pressure is applied. All the distillate is first recirculated to the reactor. The DMC content can be determined by taking a sample from the distillate stream. Depending on the optimisation target (DMC yield or STY), a reflux ratio of 100% can be employed until a minimal DMC content in the distillate is achieved, or a defined time is fixed at which a changeover is made to distillation (a DMC/methanol mixture is distilled off). The residual DMC is subsequently metered in, uncapped, and any necessary correction to the stoichiometry is made by adding further amounts of the diol components and by a renewed, brief transesterification.
Suitable aliphatic diols preferably have 3 to 20 C atoms in their chain. Examples include 1,7-heptanediol, 1,8-octanediol, 1,6-hexanediol, 1,5-pentanediol, 1,4-butanediol, 1,3-butanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2-methylpentanediol, 2,2,4-trimethyl-1,6-hexanediol, 3,3,5-trimethyl-1,6-hexanediol, 2,3,5-trimethyl-1,6-hexanediol, cyclohexane-dimethanol, and others, as well as mixtures of different diols.
Addition products of diols with lactones (ester diols) can also be used, such as caprolactone, valerolactone etc., as can mixtures of diols with lactones, wherein it is not necessary initially to transesterify a lactone and diols.
Moreover, addition products of diols with dicarboxylic acids can also be used, such as: adipic acid, glutaric acid, succinic acid, malonic acid, etc., or esters of dicarboxylic acids and also mixtures of diols with dicarboxylic acids or with esters of dicarboxylic acids, wherein it is not necessary initially to transesterify a dicarboxylic acid and diols.
Polyether polyols can also be used, such as polyethylene glycol, polypropylene glycol and polybutylene glycol, as can polyether polyols which are obtained by the copolymerisation of ethylene oxide and propylene oxide for example, or polytetramethylene glycol which is obtained by the ring-opening polymerisation of tetrahydrofuran (THF).
Mixtures of different diols, lactones and dicarboxylic acids can be used.
1,6-hexanediol, 1,5-pentanediol and/or mixtures of 1,6-hexanediol and caprolactone are preferably used in the process according to the invention.
xcex5-caprolactone esters are preferably formed in situ, without prior reaction, from the raw materials during the production of oligocarbonate diol.
In principle, all soluble catalysts which are known for transesterification reactions can optionally be used as catalysts (homogeneous catalysis), and heterogeneous transesterification catalysts can also be used. The process according to the invention is preferably conducted in the presence of a catalyst.
Hydroxides, oxides, metal alcoholates, carbonates and organometallic compounds of metals of main groups I, II, III and IV of the periodic table of the elements, of subgroups III and IV, and elements from the rare earth group, particularly compounds of Ti, Zr, Pb, Sn and Sb, are particularly suitable for the process according to the invention.
Suitable examples include: LiOH, Li2CO3, K2CO3, KOH, NaOH, KOMe, NaOMe, MeOMgOAc, CaO, BaO, KOt-Bu, TiCl4, titanium tetraalcoholates or terephthalates, zirconium tetraalcoholates, tin octoate, dibutyltin dilaurate, dibutyltin, bistributyltin oxide, tin oxalate, lead stearate, antimony trioxide, zirconium tetraisopropylate, etc.
Aromatic nitrogen heterocycles can also be used in the process according to the invention, as can tertiary amines corresponding to R1R2R3N, where R1-3 independently represents a C1-C30 hydroxyalkyl, a C4-C10 aryl or a C1-C30 alkyl, particularly trimethylamine, triethylamine, tributylamine, N,N-dimethylcyclohexylamine, N,N-dimethyl-ethanolamine, 1,8-diaza-bicyclo-(5.4.0)undec-7-ene, 1,4-diazabicyclo-(2.2.2)octane, 1,2-bis(N,N-dimethyl-amino)-ethane, 1,3-bis(N-dimethyl-amino)propane and pyridine.
Alcoholates and hydroxides of sodium and potassium (NaOH, KOH, KOMe, NaOMe), alcoholates of titanium, tin or zirconium (e.g. Ti(OPr)4), as well as organotin compounds are preferably used, wherein titanium, tin and zirconium tetraalcoholates are preferably used with diols which contain ester functions or with mixtures of diols with lactones.
In the process according to the invention, the homogeneous catalyst is used in concentrations (expressed as percent by weight of metal with respect to the aliphatic diol used) of up to 1000 ppm (0.1%), preferably between 1 ppm and 500 ppm (0.05%), most preferably between 5 ppm and 100 ppm (0.01%). After the reaction is complete, the catalyst can be left in the product, or can be separated, neutralized or masked. The catalyst is preferably left in the product.
The molecular weight of the oligocarbonate diols produced by the process according to the invention can be adjusted via the molar ratio of diol to DMC, wherein the molar ratio of diol/DMC can range between 1.01 and 2.0, preferably between 1.02 and 1.8, and most preferably between 1.05 and 1.6. The aforementioned ratio, of course, describes the stoichiometry of the product, i.e., the effective ratio of diol to DMC after distilling off the DMC-methanol mixtures. The amounts of DMC which are used in each case are correspondingly larger due to the azeotropic distillation of the DMC. The calculated number-average molecular weights of the oligocarbonate diols produced by the process according to the invention then range, e.g. when 1,6-hexanediol is used as the diol component, between 260 and 15,000 g/mol, preferably between 300 and 7300 g/mol, most preferably between 350 and 3000 g/mol. If a diol of higher or lower molecular weight is used, the molecular weights of the oligocarbonate diols produced according to the invention are correspondingly higher or lower.
The process according to the invention makes it possible to produce oligocarbonate diols of formula HOxe2x80x94R1xe2x80x94[xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94R1xe2x80x94]nxe2x80x94OH which have carbon numbers from 7 to 1300, preferably from 9 to 600, most preferably from 11 to 300, in which R1 is the symbol for aliphatic diols with 3 to 50 carbon atoms, preferably 4 to 40, and more preferably from 4 to 20 carbon atoms.
The diols can additionally contain ester, ether, amide and/or nitrile functions. Diols or diols with ester functions are preferred, such as those which are obtained by the use of caprolactone and 1,6-hexanediol. If two or more diol components are used (e.g. mixtures of different diols or mixtures of diols with lactones), two adjacent R1 groups in a molecule can definitely be different from each other (random distribution).
The process according to the invention enables high quality oligocarbonate diols to be produced from DMC with good space-time yields and with a low degree of capping of their terminal OH groups.
The oligocarbonate diols which are produced by the process according to the invention can be used, for example, for the production of plastics polymers, fibres, coatings, lacquers and adhesives, e.g. by reaction with isocyanates, or for the production of epoxides, (cyclic) esters, acids or acid anhydrides. They can be used as binder vehicles, binder vehicle constituents and/or as reactive thinners in polyurethane coatings. They are suitable as components of moisture-hardening coatings, or as binder vehicles or binder vehicle constituents in solvent-containing or aqueous polyurethane coatings. They can also be used as building blocks for the synthesis of polyurethane prepolymers which contain free NCO groups, or in polyurethane dispersions.
The oligocarbonate diols which are produced by the process according to the invention can also be used for the production of synthetic thermoplastic materials such as aliphatic and/or aromatic polycarbonates, thermoplastic polyurethanes, etc.
The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified.